The detection of radioactive materials typically requires the measurement of their emitted gamma rays and/or neutrons. The emission energy is a unique signature of the radioactive material and, therefore, serves as a means of identification. Scintillator materials are a class of materials that respond to radiation through the following mechanism: (1) impinging radiation leads to ionization of atoms (of which the scintillator material is comprised) to produce electrons and holes; (2) the electrons and holes thermalize and migrate to luminescent centers; and (3) the electrons and holes recombine to produce visible photons. When scintillators are coupled to a photodetector, such as a photomultiplier tube (PMT), silicon photomultiplier (SiPM), or silicon avalanche photodiode (APD), the emitted photons are converted into electrical pulses, which can then be analyzed to detect the presence of or to determine the identity of the radiation source. Scintillator materials have been used in a variety of fields, including homeland security, high-energy physics, medical imaging, and oil-well logging.
One measure of a scintillator's performance is its energy resolution (ER), a metric on the precision with which it can resolve different energy levels. Energy resolution is determined from the photopeak of the pulse-height spectrum generated when a scintillator responds to irradiation. ER is determined by taking the quotient of the full-width at half-maximum (FWHM) and the centroid of the photopeak. High-performance scintillators have high ERs, corresponding to low numerical values of the aforementioned quotient. Another measure of scintillator performance is its ability to distinguish between gamma radiation and neutron radiation. The figure-of-merit (FOM) characterizing this attribute is determined by integrating the neutron and gamma-ray output signal traces from the photodetector using two integration windows (fast and slow). A ratio of the integrals is plotted versus the energy, and a slice containing both the gamma-ray and neutron features is projected to produce two Gaussian-like peaks. The FOM is then calculated by dividing the separation between the peak centroids by the sum of their FWHMs. The typical requirement for adequate gamma-ray/neutron discrimination is an FOM value of 1.5 or greater.